I've been using letters for years to solicit links. I use it only to go after high value sites and the snail mail makes up only a small portion of our link campaign. But it can certainly be useful. Like email requests, letters work best if you have a specific person's name to address them to.

One thing you can do is to befriend your competitor's employees on Facebook and to follow them on twitter. Don't bother with the official company Facebook page or twitter, or the senior people in the company. See what their low level employees are saying. People spill a lot on the Internet and you can pick up some good stuff this way.